Abstract: While clinical treatment of actinic keratosis by photodynamic therapy (PDT) is widely practiced, there is well-known variability in response, primarily caused by heterogeneous accumulation and PDT-induced photobleaching of the photosensitizer protoporphyrin IX (PpIX) between patients and between lesions. One of the key factors in regularizing this treatment would be to have an easily accessible indicator of PpIX present in the lesions at the time of light delivery. Described herein, a smartphone-based fluorescence imager was developed to allow simple quantitative photography of the lesions and their PpIX levels.